A honeycomb structured body comprising a pillar-shaped honeycomb block including a honeycomb unit in which a number of cells are longitudinally placed in parallel with a cell wall therebetween. The honeycomb unit comprising inorganic particles, as well as inorganic fibers and/or whiskers, and the inorganic fibers and/or whiskers increase a specific surface area of the honeycomb unit. A sealing material provided on a peripheral portion of the honeycomb block. Irregularities are formed on the peripheral face of the honeycomb structured body and the pillar-shaped honeycomb block. A least square curve is obtained by a least square method on the basis of points comprising the contour of a cross-section perpendicular to the longitudinal direction of the honeycomb structured body and the honeycomb block.